Conventionally, in a pressure type mass flow controller, there is a case where, for example, particles contained in a fluid may cause a clogging in a differential pressure generating unit such as a nozzle and an orifice so that it becomes impossible to perform a fluid control as is intended to obtain a targeted flow rate value. To prevent such a defect, there has been known a diagnostic mechanism in which a mass flow controller per se can conduct a self-diagnosis whether or not there is a clogging in a flow passage and differential pressure generating unit, as mentioned above, in which a fluid flows.
For example, the diagnostic mechanism is adapted to fully close a flow rate control valve inside the mass flow controller or an on-off valve separately provided so as not to feed a new fluid into the mass flow controller from a primary side. In this state, as shown in a graph of FIG. 10, a pressure drop amount of a fluid and a time required for the pressure drop due to an outflow of the fluid from the inside of the mass flow controller are measured so as to be compared to specified values measured in a normal condition where there is no clogging. More specifically, in a state that the fluid supply from the primary side is interrupted and the flow rate control is not performed by the flow rate control valve, there occurs a pressure drop as shown by a solid line in the graph when in a normal condition. Whereas, in the case where there is a clogging in a flow passage, etc., as shown by a dotted line in the graph, the pressure drop is made slower than that in the normal condition and the time required for lowering to a certain pressure value is changed. The diagnostic mechanism is adapted to have a configuration that performs a diagnosis whether or not a clogging exists within the flow passage and the like.